The present invention relates generally to hardsurfacing a workpiece for effecting wear resistance, and, more specifically, to hardsurfacing superalloys subject to heat induced cracking.
Large industrial gas turbine engines may be used for powering an electrical generator for producing electrical power. The engine conventionally includes a compressor for compressing air which is mixed with fuel and ignited in a combustor for generating combustion gases which are channeled through one or more turbine stages which extract energy therefrom. Each turbine stage includes a plurality of circumferentially adjoining turbine rotor blades or buckets extending radially outwardly from a rotor disk to which they are conventionally mounted. In some designs, undesirable bucket vibrations may be eliminated by providing frictional damping using interconnecting tip shrouds integrally joined to the radially outer ends of the buckets. An exemplary tip shroud is generally rectangular in plan view and forms with adjoining tip shrouds a circumferentially continuous outer shroud ring around the buckets. Adjacent tip shrouds are complementary with each other and have a generally Z-notch therein, with the middle of each notch having a face which abuts an adjacent notch face during rotary operation of the buckets. The notch faces therefore define contact surfaces which provide frictional damping to reduce or eliminate undesirable bucket vibration during operation.
Since the notch faces are contact surfaces they typically require suitable hardsurfacing treatment to effect a suitably high wear resistance layer or cladding thereon for obtaining a suitable useful life of the buckets during operation.
Modern turbine buckets are typically formed of conventionally known superalloys for maximizing strength and useful life, and are therefore expensive to manufacture. Each bucket must be suitably manufactured and pre-machined prior to hardsurface treatment, and therefore care must be taken in the hardsurface treatment in order to avoid damaging the bucket and rendering it unacceptable for turbine use. Conventional hardsurface treatments necessarily impart heat into the bucket tip shrouds which must be carefully controlled to avoid or minimize heat induced cracking or damage in the superalloy parent material.
One conventional manner of turbine tip shroud hardsurfacing uses a conventional thermal spray process in which the portions of the tip shroud not requiring hardsurfacing are covered with a protective tape, with the notch face then being coated with a suitable hardsurface material such as chromium-carbide (CrC) which is sprayed thereon using a high temperature thermal spray process. This process is labor intensive and expensive.
Another conventional process utilizes a carbon dioxide (CO.sub.2) laser which melts a suitable high wear resistance material in powder form such as conventionally known Stellite (28Cr-19.5W-5Ni). Since the superalloy parent tip shroud material and the wear resistance coatings are different materials, the various hard surface processes must be carefully qualified to ensure the formation of a suitable wear resistance coating without unacceptably damaging the parent material by introducing heat effected microcracks for example.
Since the wear resistance material is in a powder form, some of the powder is dissipated during the process and does not deposit in the region of the notch face. The lost powder is therefore wasted. And, using a powder in the CO.sub.2 laser process typically results in the deposition of the wear resistance cladding which is non-uniform in thickness and typically has a valley in the middle portion thereof which must be suitably filled during the process.
Since the final notch face must be formed within suitable relatively small tolerances on the order of mils, the hardsurfacing treatment usually provides excess cladding so that it may be machined in a post operation to the required dimensions of the Z-notch. If insufficient hard surface cladding is initially formed in the different treatments, yet another process such as conventional tungsten inert gas (TIG) welding is then used to add additional hardsurface material to the notch face to suitably increase its size for machining to the required final dimensions. If heat induced cracks or other defects are detected in the hardsurfacing cladding or in the heat affected zone of the parent tip shroud material, a manual, hand TIG repair process is typically used. Accordingly, it is desirable to provide an improved hardsurfacing process for applying high wear resistance coatings to a superalloy turbine bucket tip shroud.